Is there a way in Dorico to make arpeggio signs invisible? I want my harp to have arpeggios in playback, but I don’t want arpeggio signs to be in the score as the player would know to roll chords slightly.
Yes, this can be done. Input the arpeggio symbol, then select it. Go to Properties and scale it to, say 2 - it won’t appear, but will still playback correctly.
You could also produce the effect you want in playback without any notation by changing the played durations of the notes in Play mode (v3.5 and earlier) or in the key editor (v4).
For an example, see here.
I can’t see the arpeggio up or down when I type it into the notes in dorico 5.1.
I see that dorico moves the notes to make room for the arpeggio, but it’s like they’re invisible. The scale is 100%… What am I doing wrong? I have a PC.
Welcome to the forum, Hans Jakob!
I understand that your problem is quite the opposite of the one that was dealt with in this thread… You maybe should have started your own 
It strikes me as a problem with the font. Which musical font are you using? Have you tried and modified that setting (Library menu>Music font) to a Bravura or Sebastian setting? Those are both fonts that come with Dorico 5 (If I am not mistaken) and Bravura is the SMuFL reference, it has all the glyphs.
It’s possible to have problems with other musical fonts (Finale Maestro, November 2, etc. have been known to have glyphs missing, which makes their usage quite unpredictable)